Monostate pattern #85

monostate/etc/MonoState.ucls

+<?xml version="1.0" encoding="UTF-8"?>
+<class-diagram version="1.1.8" icons="true" always-add-relationships="false" generalizations="true" realizations="true" 
+  associations="true" dependencies="false" nesting-relationships="true">  
+  <class id="1" language="java" name="com.iluwatar.monostate.LoadBalancer" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="187" width="158" x="100" y="66"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="2" language="java" name="com.iluwatar.monostate.Server" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/Server.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="176" y="372"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <class id="3" language="java" name="com.iluwatar.monostate.Request" project="monostate" 
+    file="/monostate/src/main/java/com/iluwatar/monostate/Request.java" binary="false" corner="BOTTOM_RIGHT">    
+    <position height="-1" width="-1" x="329" y="352"/>    
+    <display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+      sort-features="false" accessors="true" visibility="true">      
+      <attributes public="true" package="true" protected="true" private="true" static="true"/>      
+      <operations public="true" package="true" protected="true" private="true" static="true"/>    
+    </display>  
+  </class>  
+  <dependency id="4">    
+    <end type="SOURCE" refId="1"/>    
+    <end type="TARGET" refId="3"/>  
+  </dependency>  
+  <dependency id="5">    
+    <end type="SOURCE" refId="2"/>    
+    <end type="TARGET" refId="3"/>  
+  </dependency>  
+  <association id="6">    
+    <end type="SOURCE" refId="1" navigable="false">      
+      <attribute id="7" name="servers"/>      
+      <multiplicity id="8" minimum="0" maximum="2147483647"/>    
+    </end>    
+    <end type="TARGET" refId="2" navigable="true"/>    
+    <display labels="true" multiplicity="true"/>  
+  </association>  
+  <classifier-display autosize="true" stereotype="true" package="true" initial-value="false" signature="true" 
+    sort-features="false" accessors="true" visibility="true">    
+    <attributes public="true" package="true" protected="true" private="true" static="true"/>    
+    <operations public="true" package="true" protected="true" private="true" static="true"/>  
+  </classifier-display>  
+  <association-display labels="true" multiplicity="true"/>
+</class-diagram>
\ No newline at end of file

monostate/index.md

+---
+layout: pattern
+title: MonoState
+folder: monostate
+permalink: /patterns/monostate/
+categories: Creational
+tags: Java
+---
+
+**Intent:** Enforces a behaviour like sharing the same state amongst all instances.
+
+![alt text](./etc/monostate.png "MonoState")
+
+**Applicability:** Use the Monostate pattern when
+
+* The same state must be shared across all instances of a class.
+* Typically this pattern might be used everywhere a SingleTon might be used. Singleton usage however is not transparent, Monostate usage is.
+* Monostate has one major advantage over singleton. The subclasses might decorate the shared state as they wish and hence can provide dynamically different behaviour than the base class.
+
+**Typical Use Case:**
+
+* the logging class
+* managing a connection to a database
+* file manager
+
+**Real world examples:**
+
+Yet to see this.

monostate/pom.xml

+<?xml version="1.0"?>
+<project xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns="http://maven.apache.org/POM/4.0.0"
+    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
+  <modelVersion>4.0.0</modelVersion>
+  <parent>
+    <groupId>com.iluwatar</groupId>
+    <artifactId>java-design-patterns</artifactId>
+    <version>1.6.0</version>
+  </parent>
+  <artifactId>monostate</artifactId>
+  <dependencies>
+    <dependency>
+      <groupId>junit</groupId>
+      <artifactId>junit</artifactId>
+      <scope>test</scope>
+    </dependency>
+  </dependencies>
+</project>

monostate/src/main/java/com/iluwatar/monostate/App.java

+package com.iluwatar.monostate;
+
+public class App {
+	public static void main(String[] args) {
+		LoadBalancer loadBalancer1 = new LoadBalancer();
+		LoadBalancer loadBalancer2 = new LoadBalancer();
+		loadBalancer1.serverequest(new Request("Hello"));
+		loadBalancer2.serverequest(new Request("Hello World"));
+	}
+
+}

monostate/src/main/java/com/iluwatar/monostate/LoadBalancer.java

+package com.iluwatar.monostate;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class LoadBalancer {
+  private static List<Server> servers = new ArrayList<>();
+  private static int id = 0;
+  private static int lastServedId = 0;
+
+  static {
+    servers.add(new Server("localhost", 8081, ++id));
+    servers.add(new Server("localhost", 8080, ++id));
+    servers.add(new Server("localhost", 8082, ++id));
+    servers.add(new Server("localhost", 8083, ++id));
+    servers.add(new Server("localhost", 8084, ++id));
+  }
+
+  public final void addServer(Server server) {
+    synchronized (servers) {
+      servers.add(server);
+    }
+
+  }
+
+  public final int getNoOfServers() {
+    return servers.size();
+  }
+
+  public void serverequest(Request request) {
+    if (lastServedId >= servers.size()) {
+      lastServedId = 0;
+    }
+    Server server = servers.get(lastServedId++);
+    server.serve(request);
+  }
+
+
+}

monostate/src/main/java/com/iluwatar/monostate/Request.java

+package com.iluwatar.monostate;
+
+public class Request {
+  public final String value;
+
+  public Request(String value) {
+    super();
+    this.value = value;
+  }
+}

monostate/src/main/java/com/iluwatar/monostate/Server.java

+package com.iluwatar.monostate;
+
+public class Server {
+  public final String host;
+  public final int port;
+  public final int id;
+  public Server(String host, int port, int id) {
+    this.host = host;
+    this.port = port;
+    this.id = id;
+  }
+  public String getHost() {
+    return host;
+  }
+  public int getPort() {
+    return port;
+  }
+  public final void serve(Request request) {
+    System.out.println("Server ID "+id + "  processed request with value  "+request.value);
+  }
+}